A liquid metal cooled nuclear reactor needs to shut down a fission reaction of the fuel and reach low temperatures in order to handle emergency or maintenance. The reactor is commonly shut down by inserting a safety rod into the reactor core and removing neutrons from its fuel. Unfortunately, also after the reactor is shut down, residual decay heat from the reactor core lasts for a certain time. Accordingly, the temperature of a liquid metal coolant inside a reactor vessel does not lower immediately. Thus, the residual decay heat should be removed for maintenance work after the shutdown of the reactor.
The liquid metal coolant and the reactor construction have large heat capacity to assist the dissipation of the residual decay heat. The decay heat accumulated in the liquid metal coolant is transferred from the reactor vessel to the containment, and then carried away by a Reactor Vessel Auxiliary Cooling System (RVACS).
This prevents the reactor vessel and the containment made commonly of SUS from their strength degradation due to the exposure to high temperatures over a long period of time. A concrete silo arranged outside the reactor vessel and the containment is also prevented from its characteristic change and going brittle.
It has been disclosed that perforated flow channels are provided to the wall of a flow guide plate in a liquid metal cooled nuclear reactor in order to enhance the removal of such residual decay heat. An art is also disclosed, which relates to enhancement of the heat removal for a containment of a nuclear reactor by wetting the outer surface of the containment with water, although the art has not been applied to a liquid metal cooled nuclear reactor.
Meanwhile, heat generated during normal operation of a nuclear reactor and residual decay heat of the nuclear reactor are conducted to a Reactor Vessel Auxiliary Cooling System (RVACS) by radiation across the gap between the reactor vessel and the contentment of the nuclear reactor; and by heat conduction and convection of the inactive gas sealed in the gap.
Furthermore, the heat conduction and the convection have less contribution whereas the radiation has dominant contribution to the heat transfer across the gap between the reactor vessel and the containment. For that reason, the outer wall of the reactor vessel and the inner wall of the containment are surface-treated to have a high radiation factor, such that the heat-transfer efficiency increases owing to the radiation.
In fact, there exists a large temperature difference between the reactor vessel and the containment, showing a low heat-transfer efficiency therebetween in the heat removal with the reactor vessel auxiliary cooling system (RVACS).